1. Field of the Invention
This invention generally relates to systems and methods for blocking specular reflection and suppressing modulation from periodic features on a specimen. Certain embodiments relate to a system configured to block specular reflection and suppress modulation from periodic features, which are periodic in at least one dimension, formed on a specimen in an image of the specimen.
2. Description of the Related Art
The following description and examples are not admitted to be prior art by virtue of their inclusion in this section.
Fabricating semiconductor devices such as logic and memory devices typically includes processing a substrate such as a semiconductor wafer using a large number of semiconductor fabrication processes to form various features and multiple levels of the semiconductor devices. For example, lithography is a semiconductor fabrication process that involves transferring a pattern from a reticle to a resist arranged on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated in an arrangement on a single semiconductor wafer and then separated into individual semiconductor devices.
Inspection processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield in the manufacturing process and thus higher profits. Inspection has always been an important part of fabricating semiconductor devices such as integrated circuits. However, as the dimensions of semiconductor devices decrease, inspection becomes even more important to the successful manufacture of acceptable semiconductor devices because smaller defects can cause the devices to fail. For instance, as the dimensions of semiconductor devices decrease, detection of defects of decreasing size has become necessary since even relatively small defects may cause unwanted aberrations in the semiconductor devices.
One obvious way to improve the detection of relatively small defects is to increase the resolution of an optical inspection system. One way to increase the resolution of an optical inspection system is to decrease the wavelength at which the system can operate. As the wavelengths of inspection systems decrease, incoherent light sources are incapable of producing light with sufficient brightness. Using light sources that have sufficient brightness is important to successful inspection since using a light source with relatively low brightness can reduce the sensitivity of the inspection system. In particular, when using a relatively low brightness light source, the signal-to-noise ratio of output generated by the inspection system may be too low for accurate defect detection. To mitigate the effects of a low brightness light source on the output of an inspection system, the throughput of inspection may be reduced to allow enough light to be collected. Obviously, reduced throughput for inspection is highly undesirable.
Accordingly, for inspection systems that are designed to operate at shorter wavelengths, a more suitable light source is a laser light source that can generate relatively bright light at relatively small wavelengths. However, laser light sources generate coherent light. Such light is disadvantageous for inspection since coherent light can produce speckle in images of a specimen generated by the system. Since speckle is a source of noise in the images, the signal-to-noise ratio of output generated by the inspection system will be reduced by speckle. Therefore, many illumination systems have been developed for inspection applications that reduce the speckle of light from laser light sources.
Coherent light is also disadvantageous for use in imaging-based inspection systems since coherent light can produce ringing in images generated by the inspection systems. In particular, coherent light will produce relatively sharp transitions in the images instead of relatively smooth transitions. These relatively sharp transitions can produce artifacts in inspection images that will increase the difficulty of detecting defects in the inspection images. Therefore, for inspection applications, many illumination systems have been developed that reduce the coherence of the light generated by a coherent light source before the light impinges on the specimen.
Inspection of patterned wafers is also becoming increasingly important. In particular, inspection of patterned wafers allows detection of pattern-related defects and allows detection of defects on patterned product wafers, which may provide better inspection results for process control and monitoring than inspection of monitor wafers. However, inspection of patterned wafers is difficult due, at least in part, to the reflection, scattering, and diffraction from patterned features on the wafers, which may dramatically reduce the sensitivity of the inspection to defects on the wafers.
Fourier filtering is often used to block specular reflection from patterned features on specimens such as wafers. In particular, Fourier filtering suppresses modulation in the images of specimens on which a periodic array of features is formed. However, Fourier filtering advantageously preserves modulation in the image due to non-periodic features or “defects” in the periodic array of features. Fourier filtering also improves the signal-to-noise ratio of output corresponding to defects in periodic arrays of features in a number of ways. For instance, Fourier filtering allows the intensity of a defect image to be increased by preventing saturation of the sensor with the periodic array image. In addition, Fourier filtering can eliminate photon (shot) noise as a significant noise source. Furthermore, Fourier filtering can substantially eliminate the need for using cell-to-cell image subtraction for defect detection thereby eliminating noise sources associated with cell-to-cell subtraction such as image alignment errors and increased electronic and sensor noise.
A number of different existing Fourier filtering techniques are currently used. For example, some currently used Fourier filtering techniques are performed using spatially coherent illumination. Some such techniques use a single point fill of the illumination aperture. Specular reflection and diffraction from repeating arrays are blocked in the imaging aperture. Such techniques also typically require relatively narrow spectrum illumination to keep the diffracted light localized.
Another currently used Fourier filtering technique is dark field (Edge Contrast (EC)) imaging. In such techniques, an illumination ring is positioned outside of the imaging aperture. Specular reflection is blocked by a smaller imaging aperture. Diffracted orders from arrays of repeating periodic features having sufficiently small pitches are located outside of the imaging aperture.
An additional currently used Fourier filtering technique is one-dimensional (1D) dark field (DF) imaging. In such techniques, specular reflection may be blocked by an imaging aperture. Substantially all diffraction in one direction may be blocked by the imaging aperture. In addition, light diffracted into other directions from arrays of repeating periodic features having sufficiently small pitches are blocked by the imaging aperture. Examples of methods and systems for 1D DF imaging are illustrated in commonly assigned U.S. patent application Ser. No. 11/228,584 by Zhao et al., filed Sep. 16, 2005, which is incorporated by reference as if fully set forth herein.
There are, however, a number of limitations to the currently used techniques for Fourier filtering described above. For example, Fourier filtering using spatially coherent illumination has a number of limitations. In particular, laser sources are typically used in such techniques to provide illumination in a relatively narrow spectrum with sufficient brightness. In addition, relatively high power densities are generated on optical surfaces near pupil planes. A relatively high degree of spatial coherence also increases noise from wafer roughness and stray light. Furthermore, the presence of blocking features within the imaging aperture induces ringing in the images of non-periodic features on the specimen.
Dark field (EC) imaging also has a number of limitations. For example, ring illumination limits the numerical aperture (NA) available for imaging particularly in a through-the-lens illumination configuration. In addition, only arrays that include periodic features having pitches that are less than about λ/(2 NAimg) in all directions may be suppressed (where λ is the wavelength of illumination and NAimg is the imaging NA). Furthermore, a limited imaging NA may reduce capture of scattered light from defects and may reduce image resolution.
1D DF imaging also has a number of limitations. For instance, the imaging NA in one dimension is typically limited to about ½ of the objective NA. In addition, only arrays of periodic features having pitches that are less than about λ1/(2 NAimg) in the narrow NA direction may be fully suppressed. Furthermore, the limited imaging NA in one dimension may reduce capture of scattered light from defects and may reduce image resolution.
Accordingly, it would be advantageous to develop systems and methods for blocking specular reflection and suppressing modulation from periodic features formed on a specimen in an image of the specimen, which do not have one or more of the limitations described above.